Membranes charged

BMI/Epoxy resins [COMPOSITE MATERIALS - POLYTffiR-MATRM - THERMOSETS] (Vol 7) Electrically charged membrane... 

The fourth fully developed membrane process is electrodialysis, in which charged membranes are used to separate ions from aqueous solutions under the driving force of an electrical potential difference. The process utilizes an electrodialysis stack, built on the plate-and-frame principle, containing several hundred individual cells formed by a pair of anion- and cation-exchange membranes. The principal current appHcation of electrodialysis is the desalting of brackish groundwater. However, industrial use of the process in the food industry, for example to deionize cheese whey, is growing, as is its use in poUution-control appHcations. 

Modification of the membranes affects the properties. Cross-linking improves mechanical properties and chemical resistivity. Fixed-charge membranes are formed by incorporating polyelectrolytes into polymer solution and cross-linking after the membrane is precipitated (6), or by substituting ionic species onto the polymer chain (eg, sulfonation). Polymer grafting alters surface properties (7). Enzymes are added to react with permeable species (8—11) and reduce fouling (12,13). 

Polyelectrolyte complex membranes are phase-inversion membranes where polymeric anions and cations react during the gelation. The reaction is suppressed before gelation by incorporating low molecular weight electrolytes or counterions in the solvent system. Both neutral and charged membranes are formed in this manner (14,15). These membranes have not been exploited commercially because of then lack of resistance to chemicals. 

Electrically assisted transdermal dmg deflvery, ie, electrotransport or iontophoresis, involves the three key transport processes of passive diffusion, electromigration, and electro osmosis. In passive diffusion, which plays a relatively small role in the transport of ionic compounds, the permeation rate of a compound is deterrnined by its diffusion coefficient and the concentration gradient. Electromigration is the transport of electrically charged ions in an electrical field, that is, the movement of anions and cations toward the anode and cathode, respectively. Electro osmosis is the volume flow of solvent through an electrically charged membrane or tissue in the presence of an appHed electrical field. As the solvent moves, it carries dissolved solutes. 

Many related processes use charged membranes and/or EMF. Electrodialytic water dissociation (water splitting), diffusion dialysis, Donnan dialysis, and electrolysis are related processes. Electrowsis (chlorine-caustic) is a process of enormous importance much of which is processed through very special membranes. 

Electrodialysis Electrostatically charged membranes (cation and anion) Electrical potential Electrostatic diffusion... 

A. Yarshuk, E. Staude. Charged membranes for low pressure reverse osmosis properties and applications. Desalination 55 115, 1992. 

Concentration of Electrolyte Myer and Sievers"" applied the Donnan equilibrium to charged membranes and developed a quantitative theory of membrane selectivity. They expressed this selectivity in terms of a selectivity constant, which they defined as the concentration of fixed ions attached to the polymer network. They determined the selectivity constant of a number of membranes by the measurement of diffusion potentials. Nasini etal and Kumins"" extended the measurements to paint and varnish films. 

Disjoining pressure was attributed in Ref 54 to the combined effect of van der Waals attraction and long-range electrostatic repulsion between similarly charged membrane surfaces. 

Figure 7.28b shows that membrane retention is very systematically increased for almost all of the weak bases. This is a general pattern for bases with any of the negatively charged membrane models, and is probably best explained by the increased electrostatic attractions between the drugs and the membranes. Still, all retentions are below 50%, due to the offsetting sink condition created in the acceptor wells. 

D Kirstein, H Braselmann, J Vacik, J Kopecek. Influence of medium and matrix composition on diffusivities in charged membranes. Biotech Bioeng 27 1382-1384, 1985. 

A number of reports have appeared concerned with the adsorption of purines at a dropping mercury electrode 77"80> but these are confined to studies at potentials far removed from those where electrochemical oxidation occurs. More recently some qualitative studies on the adsorption of certain purines at the PGE have appeared with a view to understanding the adsorption of these compounds at positively charged electrodes. Since many biological reactions occur at charged membrane or ribosomal surfaces it is of considerable interest to investigate these phenomena. 

Ohshima, H. and Ohki, S. (1985). Donnan potential and surface-potential of a charged membrane, Biophys. J., 47, 673-678. 

SORPTION OF HIOCS TO CHARGED MEMBRANES VESICLES AND BIOLOGICAL MEMBRANES... 

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